With the rapidly growing trend toward cordless and more portable electronic equipment such as audio-visual equipment or personal computers, there is an increasing demand that secondary batteries be smaller and more lightweight with higher energy density as the driving power source. Among such secondary batteries, lithium ion secondary batteries including lithium as an active material particularly have high voltage and high energy density.
Of conventional lithium ion secondary batteries, wound-type batteries are predominant. Wound-type batteries have an electrode plate assembly in which a positive electrode plate and a negative electrode plate are wound with a separator interposed between the electrode plates. The positive electrode plate comprises a positive electrode active material part and a positive electrode current collector carrying the positive electrode active material part, and the negative electrode plate comprises a negative electrode active material part and a negative electrode current collector carrying the negative electrode active material part. The positive electrode active material part comprises a positive electrode active material capable of absorbing or desorbing a lithium ion during charge and discharge, and the negative electrode active material part comprises a negative electrode active material capable of absorbing or desorbing a lithium ion during charge and discharge. The electrode plate assembly is accommodated with an electrolyte in a battery case.
In the case of wound-type batteries, when the electrode plates are wound, the electrode plates and the separator are deformed where they are bent or curved. Such deformation may trigger an internal short-circuit. Also, deviation in position of the separator upon the winding of the electrode plates may induce an internal short-circuit.
In view of the above points, the inventors of the present invention have focused on eliminating the main cause of the internal short-circuit in wound-type lithium ion secondary batteries.
The internal short-circuit may be classified into the following four types:
(1) A short-circuit between the positive electrode active material part and the negative electrode active material part.
(2) A short-circuit between the positive electrode active material part and the negative electrode current collector.
(3) A short-circuit between the positive electrode current collector and the negative electrode active material part.
(4) A short-circuit between the positive electrode current collector and the negative electrode current collector.
The inventors of the present invention observed the presence or absence of a spark and ignition by decomposing overcharged batteries into respective parts and bringing the respective parts in direct contact with each other. As a result, in short-circuit types (1) and (2), neither spark nor ignition occurred, but in short-circuit type (3), a spark and ignition occurred. In short-circuit type (4), although a spark occurred, ignition did not take place.
These results indicate the following:
In short-circuit types (1) and (2), the resistance of the positive electrode active material itself is large, so that a large current does not flow even if the positive electrode active material part and the negative electrode active material part are short-circuited.
In short-circuit type (4), a spark is caused by the short-circuit between the conductive current collectors. However, since the current collectors themselves are not flammable, the short-circuit does not result in ignition.
In short-circuit type (3), since the positive electrode current collector and the negative electrode active material are both conductive, the short-circuit between them causes a spark, resulting in ignition of the flammable negative electrode active material.
The above observations suggest that short-circuit type (3) is the main cause of the internal short-circuit. The short-circuit type (3) actually takes place when conductive burrs come in contact with the surface of the negative electrode active material part. The burrs are produced when the positive electrode current collector is slit.
Recently, there has been proposed a stacked type lithium ion secondary battery comprising an electrode plate assembly in which a positive electrode plate and a negative electrode plate are stacked with an electrolyte layer, instead of a conventional separator, interposed between the electrode plates. With respect to such batteries that the electrolyte layer is integrated to the positive electrode plate and the negative electrode plate, Japanese Laid-Open Patent Publication No. 2000-30742, for example, proposes to prevent the short-circuit by regulating the dimensions of the electrode plates and the separator. However, the production process of such a stacked/integrated type electrode plate assembly is generally complicated, and heightening the capacity is also difficult. Further, from the viewpoint of making the battery more lightweight, Japanese Laid-Open Patent Publication No. Hei 9-213338, for example, proposes to use a current collector comprising a resin film such as polyethylene terephthalate and a metal deposited film formed on the surface of the resin film in such an electrode plate assembly.